Polyvinyl butyral resin is excellent in adhesion, heat resistance, transparency or the like, and therefore is widely used in various fields of applications as represented by, for example, coating materials, adhesive agents, binders, and various molded products. The polyvinyl butyral resin is produced and obtained as powder with a low apparent density by reacting polyvinyl alcohol with butyl aldehyde in water and/or an organic solvent in the presence of an acid catalyst, followed by performing the steps of washing, devolatilizing and drying. Since the polyvinyl butyral resin powder has a low apparent density, there has been a problem that it requires excessive volume of the storage for storing the powder for its weight, and furthermore, the transportation cost increases. Further, the polyvinyl butyral resin powder is stirred up when processed, thereby causing workability or working environment to deteriorate, and leading to a reduction in productivity.
In order to achieve a polyvinyl butyral resin with a high apparent density and the favorable handleability, pelletizing of polyvinyl butyral resin is known. For example, JP 2005-60535 A discloses a method for producing a polyvinyl butyral resin pellet including the steps of adjusting the water content of the polyvinyl butyral resin obtained by butyralizing polyvinyl alcohol with butyl aldehyde to 10 to 60 wt %, melting and kneading it by means of a melt extruder that has at least one vent portion for devolatilization equipped with vacuum means, and cutting it after extrusion into water (see claims, and [0001] and [0032] in the patent document). In this method, a polyvinyl butyral resin in the form of a wet cake having a high water content is melted and kneaded. As it is obvious from the below-described Comparative examples 1 to 2, there is a problem that an odor is given off when the polyvinyl butyral resin pellet is produced and handled.
WO 2005/019312 A1 discloses a polyvinyl butyral resin pellet that is processed into a desired shape by extruding polyvinyl butyral resin obtained by butyralizing polyvinyl alcohol with butyl aldehyde into the form of a strand by means of a melt extruder and then cutting it (see claims and [0030] in the patent document). However, there is no description about the water content of the polyvinyl butyral resin as a raw material, and neither method for reducing butyl aldehyde and 2-ethyl-2-hexenal contained in the resin pellet nor the content of butyl aldehyde and 2-ethyl-2-hexenal has been clarified.
Meanwhile, JP 2006-47974 A discloses a polyvinyl butyral resin for a heat developable photosensitive material having a content of 60 ppm or less of 2-ethyl-2-hexenal (see claims and [0006] in the patent document). According to this patent document, the content of 2-ethyl-2-hexenal is reduced because the neutralization following the reaction between polyvinyl alcohol and butyl aldehyde is carried out without high temperatures nor high alkali conditions, thereby inhibiting the formation of 2-ethyl-2-hexenal, or alternatively, only the polyvinyl butyral resin powder that has passed through a 60-mesh sieve is washed with a large amount of distilled water, thereby increasing washing efficiency. There is a description about how to reduce 2-ethyl-2-hexenal contained in the polyvinyl butyral resin powder in this patent document, however, nothing is revealed about how to reduce butyl aldehyde contained in the polyvinyl butyral resin. Further, there is no description about pelletizing the polyvinyl butyral resin powder by melt-extruding and cutting.